SUMPRODUCT() and For Each loops in Power Query: Implementations of List.Accumulate. Part 2

In my mind List.Accumulate is one of most undocumented and most underestimated functions in Power Query ‘M’ language. In this post I would like to continue explanation of this very interesting iterative function.

The first (state) argument in accumulator sub-function has to be the same type as seed value only at first iteration and only if state argument is used in accumulator (I really see no sense in manipulations without using state argument, but you can do it, that’s why I noted it). After the first iteration the accumulator result will totally depend on what this sub-function does (as shown above).

The list elements could be used in accumulator sub-function. Could be not. You can use the list only as iteration counter – accumulator will perform its manipulations as many times as the count of list elements. This means that you can use List.Accumulate as “For Each … Next” loop (which has as many steps as a count of elements in list).

Let us try to look at how those features could be implemented.

SUMPRODUCT () in Power Query

For a starter let’s try to convert the famous Excel function, SUMPRODUCT, to Power Query function.

Why we really need it?

There are a lot of ways to implement SUMPRODUCT in Power Query. For example, we can add custom column with formula like this: = [Amount] * [Price] and then just get the sum of it. It’s easy and clear, except we need to add this helper column to get just one number.

But let suppose we have got two lists by some query manipulations and these lists are not columns in any table. We can transform them to tables, and then somehow combine those tables, perhaps with Index column, and… looks not as easy as it was with columns in one table?

How it works

Here we got two lists of the same size, MyList1 and MyList2

We’ll use one of lists as an list argument of List.Accumulate. We actually will use its elements in calculation, but also the count of list elements will be used as a counter to iterations inside our function.

As the seed argument we used a record: [Sum=0, Index =0], where Sum will be used as field for calculation and Index will be used as list’s current position counter.

We can describe the accumulator function as follows:

For Eachcurrent elementIn MyList1

Multiply current on MyList2{state[Index]} (where state[Index] is the value of Index from previous step or from seed if it is first step)

Add the result to the state[Sum] (where state[Sum] is the previous/seed value of Sum)

Increase previous/seed value of Index by 1

Next

At the end of the loop we have the result as a record of two fields: Sum and Index, as a result of Accumulate. We need only Sum field value, so we just add this filed name in square brackets at the end of row to get its value. Actually it means “give me the value of the [Sum] field from List.Accumulate result”.

SUMPRODUCT() UDF for PowerQuery/’M’

You can easily transform code from above to the one-row function for future uses:

I don’t want to take anything away from your otherwise excellent blog post, but because of the (admittedly hard-to-internalize) lazy semantics of M lists, this probably isn’t the best way to implement SUMPRODUCT. How I do it is like this:

One of the hardest things about using M is that the idioms of imperative languages — even if available — often have negative consequences. Set operations are nearly always preferable to operations on individual values. Indexing operations in particular can have surprising results with respect to performance.

Hello Curt!
Sorry for late answer and thank you for the comment and really important notes. I like your trick with adding column “on-the-fly” – it could really help in many cases.
I just want to mention that I took Sumproduct() just for example of practical use, because it is popular and not implemented in ‘M’ yet (I do not sure whether it would be implemented).
Actually we can make more complex calculations with List.Accumulate. For example, if we would like to make different transformations to list elements depending on their positions or values. The simpliest example is where list element’s position is denominator or multiplicator.
Of course, ‘M’ and Power Query not designed for this purposes (in common), but we can, thats why 🙂